1. Field of the Invention
The present invention relates to a transmission comprising a main transmission and a transfer and, more particularly, to a transmission control system for controlling such a transmission.
2. Description of the Prior Art
In a conventional automotive transmission having a main transmission gear which changes the input revolving speed, namely, the output revolving speed of the engine, and a transfer gear which changes the input revolving speed, namely, the output revolving speed of the main transmission gear, for example, a transmission having a main transmission gear comprising a plurality of speeds, a hydraulic control system for controlling the speed and a control system for controlling the hydraulic control system on the basis of input signals given thereto, such as the running speed of the vehicle and the degree of opening of the throttle valve, and a transfer gear employing the output shaft of the main transmission gear as the input shaft thereof and being capable of manual speed change, the speed of the main transmission is changed according to the running condition of the vehicle, such as the running speed of the vehicle and the degree of opening of the throttle valve, even during the shifting operation of the transfer gear, and also the transfer gear is shifted even during the shifting operation of the main transmission gear. Accordingly, the variation of the input torque of the transfer gear due to the shifting operation of the main transmission gear and the variation of the output torqe of the transfer gear due to the shifting operation of the transfer gear enhance the shock of shifting operation.
Shifting operation requires a time from the provision of a shift command till the placement of the shift valve in a position corresponding to the shift command, and a time from the placement of the shift valve in a position corresponding to the shift command till the completion of the shifting operation. When another shift command is provided while the fluid pressure supplied to the clutches and/or the brakes of the transmission is transient after the shift valve has been set in the desired position, the shift valve is changed again according to the new shift command for the new shifting operation. In such a shifting operation, since the fluid pressure supplied to the clutches and/or the brakes is transient and unstable, the new shift command makes the fluid pressure further unstable, and hence the shifting characteristics of such a transmission is unsatisfactory.